The present invention relates to reinforced cast metals and more particularly to a method for casting metals reinforced with particles or fibers or whiskers of reinforcing materials distributed throughout a metal or alloy matrix according to a desired pattern of packing density. In the castings of the present invention, the packing density distribution of the reinforcing material particles can be homogeneous throughout the metallic matrix or it can vary continuously or stepwise according to some predetermined pattern.
In the present description, reference will be made to the technique of squeeze casting which involves applying pressure over the metal in the mould before and during its solidification; it is however understood that the invention is not limited thereto and is also applicable to other casting methods in which means for causing the liquid metal introduced in the mould to fill the spaces between the reinforcing particles and embed these particles within the metal matrix is provided.
Using cast-in reinforcement materials in the casting of metals such as aluminum, magnesium, copper, zinc, lead, tin and other metals and alloys thereof is well known. For instance, documents US-A-4,576,863 and 4,590,132 disclose a light metal composite material reinforced by alumina-silica fibers.
Document GB-A-1.595.280 discloses a composite material comprising aluminum or an aluminum alloy reinforced with a felt or mat of alumina whiskers or fibers. A process for manufacturing such composite material is exemplified by the steps of:
(a) preheating a mat of whiskers or fibers of unmodified alumina in the cavity of a mould or die to a temperature in the range 250-850.degree. C., introducing into the mould or die a molten mass of aluminum or of an alloy of aluminum free from any element which reacts with alumina in the temperature range, in an amount sufficient to cover said mat with the molten mass after application of pressure thereto, the aluminum or aluminum alloy being introduced at a temperature of 650-950.degree. C., PA0 (b) directly applying to the molten mass in the mould or die sufficient mechanically-applied pressure to overcome the surface tension between said mat of whiskers or fibers and the molten mass, and thereby ensuring that the molten mass penetrates and fills the interstices of the mat, and PA0 (c) allowing the molten aluminum or alloy thereof to solidify in contact with the mat of whiskers or fibers to form a composite casting having alumina whiskers or fibers embedded in aluminum or aluminum alloy.
Document GB-A-2.106.433 also discloses reinforcing cast alloys used for pistons with a mat of whiskers or fibers of reinforcing materials such as Al.sub.2 O.sub.3 or silicon carbide. A mat of this kind composed of fibers of about 2-5 .mu.m diameter can be made by a process disclosed in document GB-A-1.489.346. The volume of the mat relative to the volume of metal in the matrix is in the range of 5-40%, preferably about 20%.
Document US-A-4,587,177 discloses metal matrix composites reinforced by mineral fibers, namely ceramic fibers. The drawing and the specification in this document indicate that in some embodiments, the packing density of the fibers is not constant throughout the metal matrix but varies according to one direction of space. Unfortunately, this document does not indicate how such variable packing density of the fibers can be achieved.
EP-A-143.330 also discloses the reinforcing of piston castings with felts or mats of fibers held together by a binder. Depending on its location within the matrix, the packing density of the fibers may vary. Although the document does not teach in detail how such variation is achieved, it may be assumed that it arises from the piling or otherwise distribution within the mould of two or more mat portions, each portion having a different packing density of the reinforcing material. This document also discloses adding to the fibers, during the preparation of the mat, additional finely divided materials such as silicon, Al.sub.2 O.sub.3, SiO.sub.2, SiC, ZrO.sub.2, Si.sub.3 N.sub.4, WC, TiC, B.sub.4 C which, in chemical combination with the piston alloy will improve the properties of the piston metal in selected areas. The size of the particulate material may be between 0.025 .mu.m and 1.0 .mu.m.
Document EP-A-131.610 discloses that in the manufacture of mats of fibers to be used as reinforcement materials in the casting of light alloy articles, the fibers are stirred into a wetting agent containing carbohydrate, for instance a chlorinated hydrocarbon mixed with a sugar such as starch, glucose, fructose, sucrose, lactose and the like; then the slurry is pressed, drained of the liquid and dried to provide a selfsupporting mat of interwoven fibers. In a variant, before drying, the fibers are oriented into an array, for instance by extrusion through a nozzle.
When this structure is used for reinforcement in the casting of alloys, the binder is dispersed by the molten metal or alloy.
Other documents disclosing alloy reinforcement with various types of fibers and particles including SiC, carbon, graphite, Si.sub.3 N.sub.4, etc., are listed hereafter: JP-A-61 000 538 (TOKAI CARBON); JP-A-60 115 360 (TOYOTA); JP-A-60 103 149 (NIPPON DENSO); JP-A-60 100 643 (IZUMI ZIDOSHA); JP-A60 083 763 (NIPPON DENSO); US-A-4.508.158 (INT. HARVESTER); JP-A-59 157 236 (SHOWA ALUMINUM); JP-A-59 145 742 (AGENCY OF IND. SCI. TECH.); JP-A-58.215.263 (TOYOTA); EP-A-45 002 (TOYOTA).
The foregoing summary of the prior art indicates that the materials used as webs, arrays or mats of fibers for casting reinforcement purposes include a wide variety of constituents, e.g. rods, staple, tow, filaments, fibers or whiskers made of materials with widely varying properties. For instance, silica and alumina fibers have moderate to low reinforcing properties and amounts (in mat form) of about 15-30% by volume of such fibers (relative to the volume of the composite) are needed for enhancing the mechanical properties of the castings by only 10-50%. Quantities over about 30% are not readily possible because the metal will no longer easily penetrate the pores of the structure even under pressure. On the other hand, SiC or Si.sub.3 N.sub.4 whiskers or fibers in comparable amounts can raise the properties by a much larger factor, e.g. 600 to 1000%. This is often useless and undesirably costly in many applications where only limited reinforcement is needed, and when it is desired to use a relatively small concentration of high strength whiskers or fibers (SiC, Si.sub.3 N.sub.4, TiC, WC, etc.). Some of these materials are relatively dense and large quantities would unduly increase the weight of the casting. The problem in this case is how to uniformly distribute in the metal matrix relatively small amounts of high strength whiskers or fibers, especially when the amount is only a few percents or less.
Normally, when the fibers or particles are supplied in mat form, the structure is self-supporting and strong enough to only undergo a minimum of deformation and variation of fiber distribution during casting of the molten metal. However, to yield a mat of sufficient rigidity, the proportion of fibers relative to total bulk volume occupied by the mat should be at least about 5-10% and is generally much more. Below this limit, the proportion of components is insufficient to maintain a self-sustaining framework and, as a consequence, an assisting bracing structure or template for supporting the fibers in a desired conformation and distribution becomes necessary.